In trampoline design, the most common springs have a total elongation of about 60% of rest lengh. E.g. a 10" spring can be stretched to 16" indefinitely without deformation. These springs are usually zinc plated, chrome plated. But rust is a problem. 10 year old springs often show significant rust and pitting.
In competition grade trampolines, they will use music wire springs (high carbon steel) These springs are typically designed with a lower spring constant, higher pre-tension, and can stretch to twice their rest length. Taht same 10 inch spring can stretch to 20 inches.
But high carbon steel rusts readily.
Given the degree of flexing, what coating systems exist to prevent the springs from rusting in an outdoor environment.
Spring stretch in a trampoline is not uniform. On rectangular tramps, the side springs in the middle third stretch most. On a round tramp, the springs in line with the fabric grid stretch the most.
My suspicion is that greater flex causes cracks in the protective layer, allowing water to get in.
If this is correct, then a thinner plating may well be more effective. I've seen "yellow zinc" plated springs. Don't know if that is a real benefit or a gimmick. But plating is an alternative.
Parts are also zinc dipped. This, I udnerstand produces a thicker coating, that for a static object provides better protection.
Paint is a common protective coating. Painting the inside of a coil spring would be tricking, especially as it needs to be kept under tension while painting and drying. I would expect taht this has to periodically redone, particularly at the points of contact at the frame and mat ring
Powder coating is an option. usually more durable than paint, but I don't know if powder coats are flexible.